It has been traditional in the art of sheet-fed printing presses to provide apparatus for supporting freshly printed sheets when transferring the sheets from one printing unit to another or when transferring the sheets from the last printing unit to a delivery sheet stacker where the sheets are stacked. Typically, a sheet transfer apparatus may be disposed between the printing units in the press and function to receive a freshly printed sheet from one impression cylinder and transfer the sheet to the next printing unit or to a sheet delivery stacker.
In sheet-fed rotary offset printing presses, for example, it is customary to transfer the sheets from the impression cylinder of one printing unit to the impression cylinder of the next unit by one or more coacting transfer cylinders, each of which is provided with grippers for engaging the leading edge of the sheet. These cylinders usually are formed with substantially continuous peripheral surfaces for supporting and controlling the movement of the sheet during its transfer between printing units.
The above described transfer apparatus has proven to be effective for transferring sheets in precise registration, but has a tendency to cause the sheets to be marked or smeared. Marking and/or smearing of a freshly printed sheet often occurs as the sheet is transferred from the impression cylinder and is immediately conveyed along a serpentine or reverse curvilinear path with its wet printed side in direct contact with the surface of the transfer cylinder. Movement of the sheet is usually so fast that the ink does not have time to set before it contacts the next transfer cylinder surface. Consequently, ink accumulates on the transfer cylinder surface and as the next sheet is transferred, it may become marked or smeared by the ink accumulation on the transfer cylinders.
Marking or smearing of the freshly printed side of the sheet is often caused by a fluttering movement of the sheet as it transfers through the above-mentioned curvilinear path from an impression cylinder to a transfer cylinder. At least slight fluttering occurs in the nip region between the impression cylinder surface and the transfer cylinder surface because of the sudden reversal in the direction of movement of the sheet and the resulting inertia forces acting on the sheet as it is pulled by the grippers through the nip region. Moreover, in many instances, the trailing end portion of the wet, printed side of the sheet may be slapped against the transfer cylinder as it is pulled through the nip region. Both the fluttering movement and the tail slap can cause marking or smearing of a freshly printed sheet.